1. Field of the Invention
The present invention relates to an ion implantation system, and more particularly, to thermal regulation of an ion implantation system to reduce the temperature therein and to minimize the risk of exposure to harmful vapors.
2. Description of the Related Art
Numerous semiconductor manufacturing processes employ ion implantation for forming a p-n junction by adding dopants (impurities), such as boron (B) and phosphorus (P) to a semiconductor substrate. Ion implantation makes it possible to accurately control the concentration and depth of impurities to be diffused into a target spot on the semiconductor substrate.
Typically, an ion implanter includes an ion source that ionizes an atom or molecule of the material to be implanted. The generated ions are accelerated to form an ion beam that is directed toward a target, such as a silicon chip or wafer, and impacts a desired area or pattern on the target. The entire operation is carried out in a high vacuum.
The trend in semiconductor devices is to become smaller and thinner. As such, these smaller and thinner requirements challenge the ability of present systems to produce high dose ion beams with the low energy required to implant a high concentration of ions at a shallow depth in the semiconductor device
Ion current (current density x area) and beam energy are the fundamental process variables for the implant step. Ion dose and implant range are the resultant device variables. Ion dose relates to the concentration of implanted ions in a semiconductor material. Moreover, the energy of the ion beam determines the depth of the implanted ions before the activation anneal step. The dose rate, and therefore, the process time is proportional to the beam current. Ideally, dose rate and beam energy would be independent process variables. This is somewhat true for high energy beams used for deep implants. However, low energy ion beams are used for shallow implants, but, for standard dopant atomic ions at low beam energies, ion current is constrained by physics limitations associated with extraction and transport losses
Present ion implanters operate best at energies from about 10 keV to about 2 MeV. Shallower implant of ions will require similar beam currents as present implanters, but at much lower energies, e.g., from about 2 keV down to hundreds of eV. However, as beam energy decreases to accommodate thinner devices, beam transport of standard ions, defined as dopants, such as boron (B+), arsenic (As+) and phosphorus (P+), becomes inefficient due to beam space charge. Beam space charge may be defined as the repelling of like charges in the ion beam causing an expansion of the ion beam during transport to the target. As a result, beam transmission is greatly reduced when the energy level is reduced.
The possibility of producing useful currents of a heavy gas phase molecular ion offers significant advantages over ion source material presently used in implanters. For example, using the heavy gas molecular ion decaborane ion (B10H14+), which has ten boron atoms has advantages for low energy, high current dopant beam transport. First, the energy for each individual boron nucleus is one tenth the energy of the ion, making it possible to extract and transport at approximately ten times the beam energy. For example, a 10 keV beam of B10H14+ would deliver dopant at less than 1 keV per boron atom. Second, the dopant current is ten times the ion current. As such, only about 1 mA of B10H114+ is needed to deliver 10 mA of boron.
Thus, it would be advantageous to provide an ion source that produces heavy ions with multiple dopant atoms per ion, at a sufficient dose (current density) to be effective as an ion in an implanter system, especially for shallow depths.
However, the use of heavy molecular ions, such as decaborane, creates some unique health and safety challenges. For example, the vapor pressure of decaborane is low at room temperature (at the threshold limit value), and can be hazardous at modestly elevated temperatures. This fact is relevant and must be addressed during routine servicing of ion implanters, and especially ion sources.
Since acceleration is accomplished by application of electric fields, particles must first be charged. Charged particles are generated by an ion source, which typically is a plasma device. Several types of ion sources are commonly used in commercial ion implantation, including a Freeman and Bernas types using thermoelectrodes and powered by an electric arc, a microwave type using a magnetron, and RF plasma sources. All of which typically operate in a vacuum. The arc sources operate at refractory temperatures.
Depending on the ion source type and usage, ion implant systems are routinely serviced on intervals of a few days to a few weeks. Due to the high cost of owning an ion implant system and the need to minimize down time, sources are frequently removed at temperatures significantly above room temperature. Toxic solids with thermally sensitive vapor pressure, like decaborane, are not routinely used at present, but the removal of a warm walled source with decaborane deposits would present a serious safety risk.
To minimize release of vaporized materials, during the cleaning of the ion source chamber or recharging the ion source material, the system must be allowed to cool before releasing the internal vacuum of the system. Opening the system before cooling to room temperature may introduce a number of serious health issues dependent upon the ion source materials. As described hereinabove, the use of decaborane, although not as hazardous as some, still creates some unique health and safety challenges. Although decaborane is reduced into harmless boron at refractory temperatures, at temperatures ranging between refractory and slightly about room temperature, the possibility of deposits on components of an ion implant system can pose a potential health hazard. As such, if the ion source chamber is not completely cooled to room temperature, or below, and decaborane deposits are present, an immediate health hazard is encountered. It should be noted that at room temperature, the decaborane health hazard is greatly reduced.
The implied cost of waiting hours for the ion source to cool to room temperature would be a significant barrier to adoption of decaborane technology in semiconductor manufacturing. Also, it is obvious that removing a decaborane ion source at elevated temperatures would create an immediate inhalation hazard to personnel. Also, evaporated decaborane could contaminate the surrounding work area.
Accordingly, it would be a significant advance in the art of ion implantation to provide a thermoregulation system that reduces the time required to cool the components of an ion implanter, thereby reducing the amount of time the implantation system is inoperative, while reducing the risk of exposure to hazardous material. In addition, implanter safety can also be improved by rendering the toxic materials into a non-toxic form.
It is therefore a principal object of the present invention to provide a thermoregulation system for an ion implanter that reduces the release of hazardous materials during servicing of the instrument or recharging the ion source.
Another object of the present invention is to reduce the time required to service an ion implanter instrument.
Still another object of the present invention is to provide a high-energy ion beam that provides shallow implantation of a dopant.
A further object of the present invention is to provide shallow implantation depth without the problems normally associated with a low energy beam, such as expanding of the ion beam and the resulting negative effects.
These and further objects are accomplished by ion implanter systems and methods disclosed herein.
One aspect of the present invention relates generally to a thermoregulation system for an ion implantation system that regulates the temperature of ion implanter components including, without limitation, the ion source, beamline components, vacuum lines, vaporizers and the like.
In one embodiment, this is accomplished by contacting at least one temperature regulating means to at least a section of an individual component of an ion implanter to control the temperature therein.
An essential element of an ion implantation system of the present invention is at least one temperature controlling means, and preferably a cooling device that contacts a component of the ion implanter. The cooling device regulates the temperature of an ion source having at least one interior chamber including, without limitation, a Bernas ion source, a Freeman ion source, or a double chamber charge exchange molecular ion source as disclosed in copending U.S. patent application Ser. No. 09/596,828, entitled xe2x80x9cIMPROVED DOUBLE CHAMBER ION IMPLANTATION SYSTEMxe2x80x9d the contents of which are herein incorporated for all purposes.
In one preferred embodiment of t he present invention, the ion source housing is connected to a cooling device to reduce the temperature of the housing structure, interior and components therein or attached thereto, to a temperature whereat the vapor pressure of the source material is considered safe and/or the risk of inhalation of vapors from the source material is acceptable.
The cooling device can be activated when servicing of the ion implanter is required or activated continuously during the ion implantation process.
In another embodiment, the ion source and other components of the ion implanter may include a temperature monitoring device to insure that the temperature is being reduced to an acceptable level while the cooling device is activated.
In yet another embodiment, a vapor monitor may be connected to the ion implanter at strategic components along a path of the generated ion beam including, without limitation, at a cool down vaporizer attachment to the ion source housing, an ion source housing, and output vacuum lines, to monitor and verify vapor concentrations of an ion source material and provide another safeguard against possible exposure to a hazardous gas.
A still further embodiment envisions a backup system to ensure that the ion source material has been rendered non-toxic. As in the case of decaborane, it is known that at temperatures above 300xc2x0 C. decaborane is rendered non-toxic because of the reduction to harmless boron and hydrogen. Thus understood, it is further contemplated that the ion source compartment comprises a heating device that may be activated to ensure that any remaining source material, such as decaborane is completely reduced to harmless boron.
Other aspects, features and embodiments of the present invention will be more fully apparent from the ensuing disclosure and appended claims.